The invention relates to the separation of magnetic particles from a composition. The invention can be used in different applications, especially in the fields of biotechnology, biochemistry, and biomedicine.
Magnetic microparticles are used as a solid phase in various applications to bind biomaterial. One advantage of microparticles is the large area of the solid phase and short diffusion lengths. The size of microparticles is generally 0.05-10 xcexcm and they are available in different materials and already activated for many applications. Magnetic particles can be moved by using a magnet.
The separation methods of magnetic particles currently used include settling a reaction vessel in a magnetic field so that particles are accumulated into a so-called pellet at the bottom of the vessel. Thereafter, the liquid which is free from particles is decanted or removed by aspiration. However, the removing of the liquid from the vessel must be carried out very carefully so as not to remove the particles at the same time.
Publication WO-86/06493 proposes a method to be used in immunoassays, in which magnetic particles and the marked complex adhered to them are separated from a liquid by using a magnetic rod and subsequently taken to be measured. The tip of the rod comprises a fixed magnet and a removable protective cover to whose outer surface the particles adhere. It is preferable to cover the protective cover with another cover after the separation and before measuring. After the measurement, the protective covers are detached together with the particles and thrown away and new covers are taken for a new separation. According to the publication, the magnet can also be an electromagnet, whereby the magnetic field can be eliminated when desired.
Publication WO-87/05536 proposes a device for separating magnetic particles, comprising, on the inside, a rod movable in a vertical boring and a magnet at the lower end thereof. The device is introduced, with the magnet in the lower position, into a liquid containing particles, whereby the particles are accumulated on the end of the rod. When the magnet is allowed into the upper position, the particles can be detached from the rod. In this way, particles can be collected and transferred from one liquid into another.
However, the disclosed separation devices and methods for magnetic particles cannot be applied very well in applications in which particles must be collected from a fairly large volume and transferred into an essentially smaller one.
Now, a separation method according to claim 1 has been invented. The other claims present some preferred embodiments of the invention.
The device according to the invention comprises an elongated protective cover that includes a movable rod comprising one rod magnet in the longitudinal direction of the cover. The proportion of the length of the rod magnet to its thickness is at least about 2:1, preferably at least about 3:1, and most preferably at least about 12:1. Both the intensity and the gradient of the magnetic field thus formed are the strongest at the end of the rod and when the magnet is in the lower position, the particles from the composition are accumulated directly on the tip of the cover. Particles can be released from the tip of the cover into a volume which is many times smaller than the original composition.
The rod magnet preferably consists of a permanent magnet and a ferromagnetic arm which is its extension.
The rod magnet is preferably sufficiently long so that the upper end of its dipole always remains above the surface of the compound. If particles are to be collected from a compound column higher than the dipole, it must be seen to that the particles from the upper part of the column are first collected on the tip so that the upper end of the dipole is constantly above the particles.
Since the upper end of the magnet can be above the compound, a more effective magnet with respect to the volume of the compound can be used, accelerating and facilitating the collection.
The device according to the invention comprises, in the tip of the protective cover, an intensive magnetic field in the direction of the cover. This is especially advantageous when particles are collected from a concentration of particles which has first been created in some other way. An additional advantage is that a high retaining strength can be provided directly in the tip from which the adhesion of the liquid particularly tends to detach particles when the protective cover is lifted off the liquid.
The tip of the protective cover is preferably provided with a sharp downward projection. This minimises the amount of liquid remaining in the tip. Typically, the tip is shaped like a cone. When transferring particles into very small vessels, the tip is preferably shaped as a cone with a concave surface.
When a ferromagnetic arm is used in the magnet, the magnet and the magnetised arm together function as a long rod magnet. The arm dissolves the gradient of the upper pole of the field, whereby the upper pole does not carry out the collection of particles. In this way, the long rod magnet can be provided at a low cost. However, even with a ferromagnetic arm, it is advantageous to use a relatively long magnet (with a length of about 1.5 . . . 10 times the thickness). The length of the magnet is preferably selected so that a maximum internal, permanent field intensity is provided for the magnet in question.
The junction between the magnet and its arm is preferably made so that the arm and the magnet come inside one another for a short length. In this way, the formation of strong gradients at the junction, which may possibly collect particles, is avoided.
The cross-section of the rod magnet can be, e.g., circular or rectangular. The circular shape is the best with respect to both manufacture and use. Indeed, the rotation of the magnet on its axis, for example, has no effect in this case. In principle, the rod can be curved to make the moving mechanisms simpler.
The shape of the protective cover on the rod can vary according to the use.
Normally, the circular shape is the most advantageous with respect to both manufacture and use. In order to increase strength, the cover can be made conical, which also facilitates the manufacture of the cover by injection moulding. The cover is preferably made of polypropylene.
The invention is best-adapted to be used for particles of about 1-10 xcexcm.